The present invention relates to the polymerization of one or more olefins in a gas phase polymerization process having a low per pass conversion. Typically in gas phase polymerization of olefins the feed stream passes through a fluidized or stirred bed of growing polymer particles. The monomer in the feed stream contacts the catalyst in the growing polymer particles and is polymerized. The unreacted monomer, ballast gas, typically nitrogen, and molecular weight control agent (typically hydrogen) optionally together with a condensable gas are recycled through a compressor and heat exchanger to cool the recycle stream and optionally condense the condensable gases. The recycle stream is then made up with additional feed stream and returned to the bed of growing polymer. The polymerization of olefins is exothermic. In gas phase polymerization the removal of heat from the bed of growing polymer tends to be a rate limiting step, for a given reactor configuration. The present invention seeks to address this issue by replacing at least 80% of the ballast gas in the feed stream with a gas having a higher heat capacity than the ballast gas. The present invention also provides integrating with crackers allowing use of dilute ethylene in a gas phase process.
U.S. Pat. Nos. 4,543,399 and 4,588,790 issued to Jenkins, III. et al. Sep. 24, 1985 and May 13, 1986, respectively, assigned to Union Carbide Corporation teach incorporating into the feed stream up to about 20 weight % of the recycle stream of a condensable gas. That is a gas which condenses when a compressed recycle stream passes through a heat exchanger prior to being recycled back to the reactor. Typically these gases are C4-6 alkanes, preferably isomers of pentane and hexane. Interestingly, Jenkins does not suggest replacing any portion of the ballast gas with either or both of a condensable gas or a gas having a higher heat capacity.
U.S. Pat. Nos. 5,462,999 and 5,436,304 to Griffin et al. issued Oct. 31, 1995 and Jul. 25, 1995, respectively, and U.S. Pat. Nos. 5,405,922 and 5,352,749 to DeChellis et al. issued Apr. 11, 1995 and Oct. 4, 1994, respectively, all assigned to Exxon Chemical Patents, Inc. all teach operating a gas phase polymerization where in the feed stream may contain from about 17.5 up to 50 weight % of a condensable gas. However, the specification still teaches the feed stream comprises xe2x80x9cinertsxe2x80x9d preferably nitrogen. No other xe2x80x9cinertsxe2x80x9d are suggested or disclosed by the above patents. The patents do not suggest that the process could be further enhanced by replacing nitrogen with a gas (which may optionally be condensable) having a higher heat capacity.
U.S. Pat. No. 5,981,818 issued Nov. 9, 1999 to Purvis et al., assigned to Stone and Webster Engineering Corp. teaches the use of dilute ethylene in a number of processes. The dilute ethylene feed may comprise from 1 up to 50 weight % of ethane. One process disclosed is the gas phase polymerization of ethylene. The disclosure cautions that for gas phase processes the dilute ethylene should comprise at least about 95 weight % of ethylene (Col. 9 lines 20-25). This is a higher ethylene content in the feed gas than in accordance with the present invention. That is the feed gas in accordance with the present invention comprises less than 95 weight % of ethylene and the dilute ethylene in accordance with the present invention comprises less than 95 weight % of ethylene.
U.S. Pat. No. 6,111,156 issued Aug. 29, 2000 to Oballa et al. discloses the use of dilute ethylene in an integrated polymerization process which has a high per pass conversion. The high per pass conversion must be greater than 85%. The per pass conversion for conventional gas phase polymerization is typically substantially less than 85% (e.g. 2-10% conversion per pass).
The present invention seeks to provide a process to increase the efficiency of a gas phase polymerization having a low per pass conversion without significantly raising the dew point of the feed stream (i.e. the condensable phase is not greater than about 25 weight %, preferably less than about 20 %, most preferably less than about 17 weight %).
The present invention also seeks to provide a means of integrating a gas phase reactor with an olefin cracker.
The present invention provides a process to increase the space time yield of a low per pass conversion gas phase polymerization of one or more C2-6 alpha olefins comprising replacing not less than 80% of the ballast gas in the feed stream to a gas phase reactor with a gas having a heat capacity greater than the heat capacity of the ballast gas and does not significantly raise the dew point of the feed stream.